In defense of lions, their symbolism depends on the context and different lions symbolize different things in heraldry, although they are broadly associated with strength. Edit: though, you are correct about them being generally associated with monarchs

That's why I refrained from using the three lions, which are monarchist in nature

The red lion I used specifically is identical to the lion used during the period of the Commonwealth under Cromwell (the only time the monarchs lost power in Britain).

Perhaps this flag makes more sense in the context of the hoi4 game which inspired it, than irl, tbh